The present invention relates to an optical head and an optical information recording/reproducing device, and more particularly to an optical head and an optical information recording/reproducing device which store informations to an optical storage medium and reproduce the informations stored in the optical storage medium, and also which are capable of detecting a radial tilt of the optical storage medium.
A recording density of the optical information recording/reproducing device is inversely proportional to the square of a diameter of a beam spot on the optical storage medium. As the diameter of the beam spot is small, then the recording density is high. The diameter of the beam spot is inversely proportional to the numerical aperture of an objective lens of the optical head. Namely, as the numerical aperture of the objective lens is high, the diameter of the beam spot is small and the recording density is high.
As the optical storage medium is tilted in a radial direction with reference to the objective lens, a distortion in shape or a deformation in shape of the beam spot is caused due to a frame aberration, whereby characteristics of the recording and reproducing operations are made deteriorated. Since the frame aberration is proportional to the third power of the numerical aperture of the objective lens, the increase in the numerical aperture of the objective lens makes narrow the acceptable margin in the radial tilt of the optical storage medium to keep the recording and reproducing characteristics. If the numerical aperture of the objective lens is increased in order to increase the recording density, then it is necessary to detect and correct the radial tilt of the optical recording medium to keep the recording and reproducing characteristics.
FIG. 1 is a schematic view illustrative of a first conventional optical head which is capable of detecting the radial tilt of the optical recording medium. This first conventional optical head is disclosed in Japanese laid-open patent publication No. 9-161293. The first conventional optical head has a semiconductor laser 121, a collimator lens 122, a diffraction optical device 123, a half mirror 124, an objective lens 125, an optical disk 126, a cylindrical lens 127, an additional lens 128 and a photo-detector 129. A laser beam is emitted from the semiconductor laser 121, and transmitted through the collimator lens 122 where the laser beam is collimated. The collimated laser beam is then transmitted through the diffraction optical device 123, where the collimated laser beam is divided into three parts, for example, 0-order light, +1-order diffracted light, and xe2x88x921-order diffracted light. The divided three lights reach the half mirror 124, wherein about 50% of the divided three lights pass through the half mirror 124 and then are transmitted through the objective lens 125, wherein the lights are condensed onto the optical disk 126. The lights are then reflected from the optical disk 126 and further transmitted through the objective lens 125 to reach the half mirror 124, wherein about 50% of the reflected three lights are reflected by the half mirror 124. The further reflected three lights are then transmitted through the cylinder lens 127 and the additional lens 128 to reach the photo-detector 129. The photo-detector 129 is positioned at an intermediate point between focal points of the cylinder lens 127 and the additional lens 128.
FIG. 2 is a plane view illustrative of the diffraction optical device of the first conventional optical head shown in FIG. 1. The diffraction optical device 123 to provide both the +1-order diffracted light and the xe2x88x921-order diffracted light with the frame aberration in the radial direction of the disk 126. A lattice direction of the diffraction optical device 123 is almost parallel to the radial direction of the disk 126. A lattice pattern of the diffraction optical device 123 is such that a left half region has a downwardly-arched pattern and a right half region has a upwardly-arched pattern, wherein the left and right regions are bounded by a center line crossing an optical axis of the diffraction optical device 123 and in a tangential direction perpendicular to the radial direction.
FIG. 3 is a plane view illustrative of an arrangement of beam spots of an alignment of tracks of the optical disk of the first conventional optical head shown in FIG. 1. Each of the tracks has a single alignment of pits. First, second and third beam spots 131, 132, and 133 correspond to the 0-order diffracted light, the +1-order diffracted light and the xe2x88x921-order diffracted light, respectively. The first, second and third beam spots 131, 132, and 133 are aligned on a single track 130. The second beam spot 132 has a right side lobe in a right side with reference to the radial direction. The third beam spot 133 has a left side lobe in a left side with reference to the radial direction.
FIG. 4 is a plane view illustrative of an arrangement of the beam spots and an alignment of patterns of photo-receiving parts of the photo-detector in the first optical head shown in FIG. 1. First, second and third beam spots 140, 141 and 142 correspond to the 0-order diffracted light, the +1-order diffracted light and the xe2x88x921-order diffracted light, respectively. The first beam spot 140 is received by divided photo-receiving areas 134, 135, 136 and 137 which are bounded by both a first dividing line crossing the optical axis and being parallel to the tangential line of the disk 126, and a second dividing line crossing the optical axis and being parallel to the radial direction. The second beam spot 141 is received by a single photo-receiving area 138. The third beam spot 142 is received by a single photo-receiving area 139. The alignment of the first, second and third beam spots 131, 132, and 133 are parallel to the tangential direction, whilst the alignment of the first, second and third beam spots 140, 141 and 142 are parallel to the radial direction perpendicular to the tangential direction due to the functions of the cylinder lens 127 and the additional lens 128.
Outputs from the photo-receiving areas 134 through 139 are represented by V134 to V139. Focus error signals are obtained by an astigmatism method, wherein an operation (V134+V137)xe2x88x92(V135+V136) is made. Track error signals are obtained by a push-pull method, wherein an operation (V134+V136)xe2x88x92(V135+V137) is made. A reproducing signal by the beam spot 131 is obtained by an operation (V134+V135+V136+V137).
The radial tilt of the disk 126 may be detectable by either one of the following two methods. First method is to obtain the radial tilt signal from the operation (V138xe2x88x92V139). Second method is to obtain the radial tilt signal from a difference in bit error rate of between a first reproducing signal by the beam spot 132 obtained from the V138 and a second reproducing signal by the beam spot 133 obtained from the V139.
If the radial tilt is detected by the above first method, then a variation of V138 and V139 with reference to the radial tilt is extremely small, for which reason it is difficult to realize a highly sensitive detection of the radial tilt. If the radial tilt is detected by the above second method, then it is necessary to measure the bit error rate in the reproducing signals, for which reason it is possible to detect the radial tilt for the reproducing only disk having already stored the signals, whilst it is impossible to detect the radial tilt for the write-enable disk.
In the above circumstances, it had been required to develop a novel optical head and a novel optical information recording/reproducing device free from the above problem.
Accordingly, it is an object of the present invention to provide a novel optical head capable of detecting a radial tilt of an optical storage medium free from the above problems.
It is a further object of the present invention to provide a novel optical head capable of a highly sensitive detection of a radial tilt of any types of optical storage mediums, for example, not only a reproducing only optical storage medium but also a write enable optical storage medium.
It is a still further object of the present invention to provide a novel optical information recording/reproducing device capable of detecting a radial tilt of an optical storage medium free from the above problems.
It is yet a further object of the present invention to provide a novel optical information recording/reproducing device capable of a highly sensitive detection of a radial tilt of any types of optical storage mediums, for example, not only a reproducing only optical storage medium but also a write enable optical storage medium.
The present invention provides an optical information recording/reproducing device which comprises a light source for emitting a light, a light transmitting system including an objective lens for focusing the emitted light onto an optical storage medium and a photo-detector system for detecting a reflected light from the optical storage medium, wherein before the light is incident into the objective lens, the light has been divided into a main beam and at least a sub-beam which are different in intensity distribution from each other, and the photo-detector system detects first and second track error signals from the main beam and the sub-beam separately, and said photo-detector system further obtains a difference in phase between the first and second track error signals in order to detect a radial tilt of the optical storage medium on the basis of the difference in phase, and further a compensation of the radial tilt is made on the basis of the detected radial tilt.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.